DESCRIPTION: Insulin-like growth factors (IGFs) I and II are key regulators of bone formation, that influence osteoblast proliferation and differentiation and function in vivo. Osteoblasts also produce IGFBPs which either inhibit or stimulate IGF activities. This proposal focuses on IGFBP-6 which has been shown to inhibit IGF-II stimulated activities in a number of cell types including human bone cells (HBCs). The principal investigator's laboratory has recently demonstrated that RA increases IGFBP-6 production through a transcriptional mechanism which requires de novo protein synthesis. Hence, this application proposes to study the transcriptional mechanism by which RA stimulates IGFBP-6 expression. Genomic clones which contain the human IGFBP-6 gene transcription unit, the 5' and 3' flanking regions, will be used to first sequence its promoter and to determine the start site of its transcription. The functional regulatory regions of the gene that are responsive to RA will be identified using promoter deletion analyses and site directed mutagenesis. Future studies will utilize this information to identify the transcription factors which regulate the transcriptional response of IGFBP-6 to RA. These studies will increase our understanding of the impact of RA on human bone physiology.